Aircraft are controlled in the air through movement of ailerons, flaps, rudders and other control surfaces. In many aircraft, the control surfaces are moved by hydraulic actuators. Often, because of dimensional constraints or the force required, more than one actuator is used to position a control surface. When hydraulic actuators operate in parallel on a single control surface, their movement must be closely synchronized. When the actuators lose synchronization, they work against each other and a condition known as "force fight" results. Force fight is detrimental because it stresses the control surface structure. The stress caused by force fight can fatigue the control surface structure and may result in premature failure.
Another reason that parallel actuators are used in aircraft to operate control surfaces is to provide redundancy. It is common to operate each actuator on an independent hydraulic circuit. This way, if a hydraulic system fails, the remaining system can still be used to position the control surface and operate the aircraft. The use of independent hydraulic systems to operate a control surface increases the instances of force fight. Force fight occurs due to differences in the components which make up the parallel systems. These differences may result from manufacturing variations in the valve's or actuator's uneven wear, obstructions in hydraulic lines or differences in the electrical or other signals which actuate hydraulic flow control valves. These differences can cause one actuator to respond faster than another resulting in undue stress to the control surface as it is moving to a final position. The problem is particularly severe when the hydraulic systems are called on to respond as rapidly as possible.
Force fight may also occur when one hydraulic system becomes inoperative. The inoperative system resists the operative system's efforts to move the control surface which again results in undue stress.
One of the approaches previously used to reduce the problem of force fight on control surfaces is to use tandem or "in-line" actuators to operate each ram or shaft which positions a control surface. This approach places the stress of the hydraulic imbalance on the common shaft rather than on the control surface. The problem with this approach is that there is a weight penalty associated with the use of redundant hydraulic systems in a single tandem assembly. In addition, because an actuator from each independent hydraulic system must be tied to each shaft or ram, such systems cannot be practically used where more than two actuators are used to position a controlled surface.
Another proposed solution to solving the problem of force fight is to employ electronic pressure sensors on the ports of the parallel actuators to monitor pressure. The sensors are connected to a computer processor which is programmed to adjust the electrical signals to the control valves to equalize the pressure. The problem with this approach is that the system has to be constantly self-adjusting. This makes the programming for such a system exceedingly complex. The required sensors and other components makes such a system expensive to implement. In addition, such a system could not prevent force fight from occurring in full control situations where maximum fluid flow to each actuator is desirable.
Thus, there exists a need for a system for preventing force fight which is more reliable, lower in weight, and less expensive to implement than prior systems. Further, there exists a need for a hydraulic pressure equalization apparatus which can be used in a system for reducing force fight on the control surfaces of aircraft.